n. a set of things that share a general characteristic
Their sizes range from 2mm to 40mm.

n. the part of the eye that focuses light on the retina
Each of your eyes has a clear curved covering, called a lens.

n. the smallest working part of a living thing
All living things are made up of cells.

n. the part of the eye that receives light and sends signals to the brain
The retina is the most complicated part of the eye; it is like a messenger that sends information to the brain.

n. a machine that takes photographs
Bring a camera. You can take photos with your friends.

n. to stop from doing something
I tried to avoid the professor after I failed the test.

n. to use and change something to make it different or more useful
Even computers can process only one piece of code at a time.

n. hard to believe; causing surprise, usually in a good way
The ballet dancer’s performance was amazing.

n. one millionth (1/1,000,000) of a second
A microsecond is one-millionth of a second.

n. the body part in the head that controls the body and thinking
Our brains can make interesting dreams when we sleep.